The present invention relates to an active matrix liquid crystal display device and, more particularly, to an active matrix liquid crystal display device having improved electrostatic protection.
FIG. 1 shows a conventional active matrix liquid crystal display device in which liquid crystal 14 is hermetically sealed in the space defined by a pair of opposed transparent base plates 11 and 12 as of glass with a spacer 13 interposed therebetween along their marginal edges. The one transparent base plate 11 has on its inside a plurality of pixel electrodes 15 each adjoined by a thin film transistor (hereinafter referred to as TFT) 16 serving as a switching element. The TFT 16 has its drain connected to the pixel electrode 15 corresponding thereto. The other transparent base plate 12 has on its inside a transparent common electrode 17 opposite the pixel electrodes 15 across the liquid crystal 14.
As shown in FIG. 2, the transparent base plate 11 has the pixel electrodes 15 square in shape and closely arranged in a matrix form, gate buses 18 formed as scanning buses in close proximity to the electrode arrays in the row direction and extending along them and source buses 19 formed as signal buses in close proximity to the electrode arrays in the column direction and extending along them. At the intersections of the gate and source buses 18 and 19 there are disposed the TFTs 16, which have their gates and sources connected to the gate and source buses 18 and 19 at their intersections and have their drains connected to the pixel electrodes 15.
When applying voltage across a pair of selected ones of the gate and source buses 18 and 19, only the associated TFT 16 is turned ON to store therethrough charges in the pixel electrode 16 connected to its drain, applying voltage across only that portion of the liquid crystal 14 sandwiched between the activated pixel electrode 15 and the common electrode 17. This renders the above-said portion of the liquid crystal 14 transparent or nontransparent to light, thus providing a selective display. The display can be erased simply by discharging the charges stored in the pixel electrode 15. Connected to the gate and source buses 18 and 19 at one or both ends thereof and provided at marginal edges of the transparent base plate 11 are terminals 20a and 20b for external connection.
The TFT's 16 have such a construction as shown in FIGS. 3 and 4. On the transparent base plate 11 the pixel electrode 15 and the source bus 19 are each formed by a transparent conductive film as of ITO and a semiconductor layer 21 as of amorphous silicon is deposited which bridges the gap between the pixel electrode 15 and the source bus 19 along their parallel-opposed marginal edges. The semiconductor layer 21 is covered with a gate insulating film 22 as of silicon nitride. On the gate insulating film 22 a gate electrode 23 is formed which extends between and partly overlaps the pixel electrode 15 and the source bus 19. The gate electrode 23 is connected at one end to the gate bus 18. Thus, those portions of the pixel electrode 15 and the source bus 19 which are opposite to the gate electrode 23 form a drain electrode 15a and a source electrode 19a, respectively. The electrodes 15a and 19a, the semiconductor layer 21, the gate insulating film 22 and the gate electrode 23 constitute the TFT 16. The gate electrode 23 and the gate bus 18 are simultaneously formed of aluminum (Al), for example. A protective layer 29 for protection of the liquid crystal 14 is deposited all over the gate electrode 23.
As depicted in FIG. 5 which is a sectional view taken on the line V--V in FIG. 3, one end portion of the pixel electrode 15 underlying the neighboring gate bus 18 extends to substantially the center of the bus 18 widthwise thereof to form an additional capacitance region 30 in the gate insulating film 22 between the extending end portion of the pixel electrode 15 and the gate bus 18. The additional capacitance region 30 is needed to supplement the electrostatic capacitance of the pixel electrode 15 to provide a large time constant composed of the electrostatic capacitance of the pixel electrode 15 and the resistance value of a channel region of the TFT 16.
In the course of manufacturing a liquid crystal display device static electricity occasionally develops, flows into a particular gate or source bus and breaks down or deteriorates many of TFT's connected thereto, resulting in what is called a line defect. A conventional solution to this problem is shown in FIG. 6, according to which during the manufacture of the display device the gate and source buses 18 and 19 are all shorted by an external short circuiting bus 31 to widely distribute the static electricity to all the buses, thereby lessening its influence. Near the end of manufacture the transparent base plate 11 is cut along the broken line l to remove the external short circuiting bus 31.
Another solution proposed so far is depicted in FIG. 7, in which an internal short circuiting bus 32 is provided and a diode circuit 33 formed by an anti-parallel connection of a pair of diodes is provided between the internal short circuiting bus 32 and one end of each of the gate and source buses 18 and 19 so that all the buses are short-circuited with respect to static electricity of a relatively high voltage in excess of threshold voltages of the diodes to thereby produce the same effect as mentioned above.
Near the end of the manufacture wires leading to the diode circuits 33 are each cut by etching as indicated by P.
With the conventional method employing the external short circuiting bus 31 shown in FIG. 6, the peripheral portion of the base plate where the short circuiting bus 31 is present is removed by means of a diamond cutter shortly before the end of the manufacturing process, but such mechanical cutting may sometimes generate static electricity which will make the display element defective.
With the method utilizing the internal short circuiting bus 32 and the diode circuits 33 as shown in FIG. 7, the wires leading to the diode circuits 33 must be cut at the point P by etching in a manner to prevent the generation of static electricity--this calls for an appreciable number of fabrication steps.
In either case, the finished display element is completely open to static electricity. Accordingly, there has been a strong demand for static electricity control measures which would be effective in the assembling of, for example, a liquid crystal television set using the finished display device as well as in the inspection of the finished display device.